ui: Move Nodes tool: drag nodes in the view with a ghost and quality colours

With the form open a drag on the mesh grabs the node under the press and
moves it in the screen plane; Alt + scroll sizes the radius. The view draws
the original edges as a ghost under the moved ones, coloured by how each
element's scaled Jacobian changed, and the sphere; the form shows the region's
numbers live. Release runs mesh.node.move (one undo step), Esc cancels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/context.rs

@@ -130,7 +130,7 @@ fn form(app: &mut ModelApp, ui: &mut egui::Ui, t: &Tokens) {
     let selected = (!app.pick.selection.is_empty()).then(|| app.pick.selection.summary());
     // While a job runs (this form's mesher, a solve) OK waits for it.
     let running = app.busy().map(str::to_string);
-    let relax_note = app.relax_note();
+    let relax_note = app.relax_note().or_else(|| app.node_move_note());
     let units = app.session.model.units();
     let session = &app.session;
     // The set Use selection will make on OK is offered with the others.